At 196 cm (6 ft 5 in), Usain Bolt is the tallest man to set a world record in the 100 meters in modern track and field. The height and mass of the fastest swimmers and runners have been increasing since the dawn of modern athletics.
Over the last century, world records have been dropping at a dramatic rate. This is certainly true in the premier track and swimming sprinting events: the 100m in track and the 100m freestyle in swim. In 1921, American sprinter Charlie Paddock set the world record in the 100m by running 10.4 seconds. By any measure that is fast, but for comparison, there were six high school athletes in the US who ran sub-10.4 seconds in the 100m during the 2014 prep track season. Today, the track world record for the 100m stands at 9.58, set by Jamaican Usain Bolt in 2009. In swimming, Hawaiian Duke Kahanamoku broke his own world record by one second in the 100m freestyle by covering the distance in 60.4 seconds in 1920. Today, Brazilian Cesar Cielo Filho holds the world record in the 100m freestyle with a time of 46.9 seconds. The figure below shows that world record speed has been increasing steadily for the last 100 years in both the 100m track sprint and 100m freestyle swim. What has lead to this dramatic increase in speed? While most of the gains have probably come from external factors, some gains are the result of changes in body composition of the athletes.
Technology has played a pivotal role in athletes' success at dropping world records over the last century. For example, the spring in todays tracks reduce energy lost with the impact of each step. The mass of track spikes have been contracting with new lightweight materials and designs. This is crucial because mass on the feet has more pendular inertia, meaning it takes more force to move the fast flying legs. Swimmers have benefited from cutting-edge swimsuits that reduce drag in the water. Furthermore, advances in training technique, sports science and nutrition have all played a role in dropping these world records. In addition, as the global population increases and a greater proportion of people have access to participate in athletics there are more athletes to draw from. Participation in athletics has been further fueled by increasingly lucrative wages for those in the upper echelons of sport. This has led to an explosion of body types in the realms of sport.
From this explosion in body types, world record holders in the premier sprinting events of track and swimming are getting heavier, taller, and more slender. A recent analysis of the body metrics of world record holders shows that their evolution of body size and shape far outpaces that of the global population (The evolution of speed, size and shape in modern athletics, 2009. Charles JD and Bejan A).
Across species, animal size is correlated with speed. This holds true on land, in air and in water. For example, the blue whale, which is the largest species to ever exist in the animal kingdom, has a maximum speed of 50 km/hr making it one of the fastest marine animals. However, it is one of the slowest marine animals relative to its size. In fact, energy expenditure is optimized when the effort expended in the horizontal direction against drag is roughly the same as the energy required to fight gravity in the vertical direction. This theory can be modeled with an equation that estimates maximum speed from body mass, which holds up well with existing data.
With regard to humans, the data clearly shows that sprint speed records in the 100m sprint on the track and the 100m freestyle in the pool are correlated with body mass. This correlation is stronger in the track sprint than freestyle swim. The figure below shows these correlations. What is perhaps most interesting is that body mass has increased in these world record athletes over the last 100 years.
Like body mass, a similar pattern has been observed in the evolution of world record athletes’ height. Height is also positively correlated with velocity. Like body mass, height has been increasing in world record holders for the past 100 years. We would expect height to increase somewhat as the height of the general population has increased. However, the height increase of champion swimmers and sprinters far outpaces the height increase of the general population. In the 20th century, the global population’s mean height increased roughly 5 cm. During that same period, the mean height of champion swimmers has increased by 11.4 cm and champion sprinters by 16.2 cm. The figure below demonstrates the correlation between height and speed in world record athletes.
The authors combined height and body mass into a single unit, slenderness, to look at how body shape has evolved in world-record sprinters and swimmers. Slenderness is proportional to the cube of height divided by the body mass. This measure of slenderness has been increasing in world record athletes over the last 100 years.
What can we learn from this study? The authors suggest that the sprinting events will continue to be dominated by the biggest athletes and that these events could be separated based on height or weight class just like boxing and wrestling. The rules of animal locomotion predicts this increase in size as the pool of men (and women) in the upper bins of human size distribution are increased. We should expect that as the global population steadies out and the upper limits of human dimension are exhausted, world records will steady out. Records are meant to be broken, but likely not at the pace we saw in the last one hundred years.